Optical Coherence Tomography (“OCT”) is a type of optical coherence-domain reflectometry that uses low coherence interferometry to perform high resolution ranging and cross-sectional imaging. In OCT systems, a light beam from a low coherence light source is split into a reference light beam and a sample light beam. A diffraction grating may be used to provide an optical path difference in one or both light beams. The sample light beam is directed onto a sample and the light scattered from the sample is combined with the reference light beam. The combination of the sample and reference light beams results in an interference pattern corresponding to the variation in the sample reflection with the depth of the sample, along the sample beam. The sample beam typically suffers a high loss of energy due to its interaction with the sample. The reference beam serves as a local oscillator to amplify the interference pattern to a detectable level and therefore must have a much higher energy level than the sample light beam. The interference pattern is detected by a photo detector, whose output is processed to generate a cross-sectional image of the sample. High resolution (less than 10 micrometer) imaging of the cross-sections of the sample by OCT is useful in biological and medical examinations and procedures, as well as in materials and manufacturing applications.
OCT based systems may be implemented with fiber optics and an optical fiber carrying the sample light beam may be incorporated into a catheter or an endoscope for insertion into internal body cavities and organs, such as blood vessels, the gastrointestinal tract, the gynecological tract and the bladder, to generate images of internal cross-sections of the cavities or organs. The sample beam is typically emitted from the distal end of the instrument, where a prism or a mirror, for example, directs the sample light beam towards a wall of the cavity. The optical fiber and the prism or mirror may be rotated by a motor to facilitate examination of the circumference of the cavity.
An example of a fiber optic OCT system is shown in U.S. Pat. No. 5,943,133 (“the '133 patent”), where sample and reference light beams are carried in respective optical fibers to a diffraction grating, which introduces an optical path difference across the light beams and also combines the sample and reference light beams. FIG. 1 is a schematic diagram of a system 10 disclosed in the '133 patent. The system includes a light source 12 optically coupled to a 50/50 beam splitter 14 through an optical fiber 16. The beam splitter 14 splits the incident light beam equally into a sample light beam and a reference light beam. The sample light beam is carried by an optical fiber 18 to a focusing lens 20, which focuses the sample light beam onto a sample 22. The optical fiber 18 may be contained within a catheter (not shown) for insertion into a body cavity, such as a blood vessel, for examination of the tissue of the wall of the cavity. Light received from the tissue is focused by the lens 20 and coupled back into the optical fiber. The received light travels back to the beam splitter 14, where it is split again. A portion of the received light is directed into another optical fiber 24, which conveys the light to a first collimator 26. The reference light beam travels through an optical fiber 28 to a second collimator 30. The first and second collimators 26, 30 direct the sample and reference light beams onto the same region of a diffraction grating 32. The diffracted, combined light beam is conjugated on the detector plane of a multi-channel linear diode array detector 34 by a conjugating 36 lens. A neutral density filter (not shown) is provided to decrease the energy in the reference beam to prevent saturation of the detector.
The sample light beam suffers a significant loss of energy due to its interaction with the sample. The second pass through the 50/50 beam splitter further reduces the already attenuated light beam. In addition, the interaction of the light beams with the diffraction grating causes a further loss in both the sample light beam and the reference light beam of about 50% of the incident light in the first order. The diffraction grating also introduces noise. As a result, the system of the '133 patent has a low signal-to-noise ratio.
Another interferometric system using a diffraction grating is described in “Nonmechanical grating-generated scanning coherence microscopy”, Optics Letters, Vol. 23, No. 23, Dec. 1, 1998. FIG. 2 is a schematic diagram of the disclosed system 50. A light source 52 provides light to a 50/50 beam splitter 54 that splits the energy in the light beam equally into a sample light beam 55 and a reference light beam 56. The sample light beam 55 is directed to a focusing lens 58 that focuses the sample light beam onto a sample 60. The light received by the focusing lens 58 from the sample 60 is returned to the beam splitter 54. The reference light beam 56 is directed to a diffraction grating 62 in a Littrow configuration, which introduces an optical path difference across the reference light beam. The diffracted reference light beam is also returned to the beam splitter 54. The sample and reference light beams are then combined in the beam splitter 54 and directed to a charge-coupled device (CCD) array 64 for detection and processing by a computer 66. The reference light beam needs to be suppressed here, as well.
Here, only the reference light beam is diffracted, making the system 50 more efficient than the system 10 of the '133 patent, shown in FIG. 1. However, the sample and reference arms in the system 50 of FIG. 2 cannot both be implemented with fiber optics. The diffraction grating introduces an optical path difference across the width of the beam. The detector is a multi-element detector at least as wide as the light beam and each element of the detector receives a portion of the beam corresponding to its position on the diffraction grating. If the reference light beam is conveyed by an optical fiber from the diffraction grating to the detector, the spatial order is lost. If the sample arm is implemented in fiber optics but the reference arm is not, the length of the open space reference arm would be inconveniently long.
In OCT systems, either the reference light beam or the sample light beam may be modulated to provide a relatively low frequency beating used as a carrier frequency. The mechanical motion may be used to scan the optical path, which essentially represents the sample depth. This motion also creates a Doppler frequency shift. A moving or oscillating mirror and a fiber stretcher, such as a piezoelectric stretcher, are commonly used for mechanically modulating the light. One or a pair of acousto-optic modulators may also be used to modulate the light beam, as described in U.S. Pat. No. 5,321,501, for example. The amplitude of the frequency of modulation is modulated by the intensity of the reflected and scattered light in the sample beam. The signal is then processed using a narrow band amplifier tuned to the frequency, to extract the intensity variation to produce an image.
In diffraction grating based interferometry using a multi-element photo detector, scanning the depth is typically not necessary because the depth is instantly projected onto the multi-element photo detector. Depending on the signal processing method, however, there may be a need for low frequency modulation. For example, if the detector is a photo diode array and heterodyne signal processing is used, low frequency modulation is required. Providing a separate modulating unit in the interferometer takes up additional space and adds to the complexity of the system. If the detector is a charge coupled device (CCD), modulation is not needed.
In prior art diffraction grating based OCT systems, the sample light beam typically passes through the beam splitter that creates the sample and the reference light beams, twice. It is therefore most efficient to use a 50/50 beam splitter that directs half of the energy from the light source into the reference beam and half of the energy into the sample beam. However, much of the energy in the reference light beam needs to be suppressed to prevent saturation of the detector. Such energy is lost in the system. The sample light beam, which suffers high loss due to its interaction with the sample as well as the second pass through the beam splitter, only receives half of the energy of the light source. The sample light beam also suffers loss and noise if it is diffracted by the diffraction grating. A more efficient diffraction grating based interferometer for use in OCT systems would be advantageous. A more efficient diffraction based interferometer, where the sample and reference light beams are carried by optical fibers, would also be advantageous.